1. Field of the Invention
This invention relates to a galvannealed steel sheet with thick plating layer having distinguished anti-powdering and anti-flaking properties and a coating weight of 45 to 90 g/m.sup.2, necessary for formings such as press forming, etc., and also to a process for producing the same.
2. Prior Art
A galvannealed steel sheet is produced by hot dip plating a steel sheet with molten zinc and heating the hot dip zinc-plated steel sheet, thereby diffusing iron from the matrix steel sheet into the zinc plating layer to conduct and alloying treatment. By the alloying treatment, iron-zinc alloy phases are formed. As compared with zinc-plated steel sheets, the galvannealed steel sheets have good paint corrosion resistance and weldability and thus are widely utilized as materials for automobiles, housing structural materials, domestic electrical appliances, etc. [Japanese Patent Application Kokai (Laid-open) Nos. 62-124266, 62-130268, etc.]
Recently, a higher corrosion resistance has been keenly desired, and galvannealed steel sheets with a thick plating layer of high coating weight are desired. However, the galvannealed steel sheets are produced by heat diffusion treatment as mentioned above, formation of Fe-Zn alloy phases having a higher iron concentration, that is, intermetallic compounds of .GAMMA. phase and .GAMMA..sub.1 phase, which are hereinafter referred to generally as .GAMMA. phase, increases in the region of the zinc plating layer at the boundary to the matrix steel sheet with increasing coating weight of the zinc plating layer. Thus, the brittle .GAMMA. phase having a higher Fe concentration is liable to form in the region of the zinc plating layer to the boundary to the matrix steel sheet and the thickness of .GAMMA. phase increases with increasing coating weight of the zinc plating layer.
On the other hand, a pure zinc phase (.eta. phase) is liable to remain without alloying in the surface region of the zinc plating layer with increasing coating weight of the zinc plating layer, and also a thick Fe-Zn alloy phase (.zeta. phase) having a relatively low Fe concentration is liable to form. With a thicker plating layer, it is thus hard to obtain a plating layer structure approximate to the ideal structure type, i.e. the structure consisting mainly of uniform .delta..sub.1 phase with a small Fe concentration gradient, which are characteristic of galvannealed steel sheet with an ordinary coating weight of less than 45 g/m.sup.2.
The thicker the phase, the more peelable the plating layer at the press forming, causing the so-called powdering to take place and press scars, etc. to develop on the products. Thus, such actual disadvantages as a decrease in the yield or a decrease in the efficiency due to increased frequency of die washing, etc. are brought about.
Thus, it has been desired to develop a technique of inhibiting the formation of .GAMMA. phase as much as possible, because the tendency to form the .GAMMA. phase is remarkable with increasing coating weight of the plating layer.
When the degree of alloying is lowered to decrease the formation of .GAMMA. phase on the other hand, the Fe-Zn alloy phase (.zeta. phase) having a relatively low Fe concentration or the zinc phase (.eta. phase) still containing a Fe solid solution is liable to remain in the surface region of the plating layer. With degreasing degree of alloying, the thickness of such phases is increased. Such tendency is remarkable with increasing coating weight of the plating layer.
When such a .eta. phase or .zeta. phase much remains in the surface region of the plating layer, the plating layer is more readily scraped by the die at the press forming, because such phases are relatively soft, resulting in the so-called flakings. The resulting flakes are accumulated around the die bead part or fall into the die, resulting also in a decrease in the yield or workability at the press forming.
It is desirable from the viewpoint of the above-mentioned problems of press forming that the ideal plating layer structure of galvannealed steel sheet consists of a uniform .delta..sub.1 phase with no Fe concentration gradient throughout the plating layer from the boundary to the matrix steel sheet to the surface region of the plating layer, but it is very difficult to obtain such a structure so long as the alloying is carried out by a heat diffusion treatment.
The above-mentioned problems are less serious with decreasing coating weight of the plating layer, because the formation of .GAMMA. phase and .zeta. phase can be reduced thereby. However, with recent expansion of application fields and consequent more stringent forming conditions, it has been more keenly desired to provide galvannealed steel sheets having much more formability than ever before. The formability can be considerably improved by reducing the coating weight of the plating layer to 30 g/m.sup.2 or less, but in order to meet the recent requirements for improving the rust-proof property at the same time, a coating weight of at least 45 g/m.sup.2, desirably 50 g/m.sup.2 or more, is required. In the actual applications, it is the most important premise obtain a corrosion resistance of desired level, and it is thus not possible to solve the problems merely by making the coating weight of the plating layer smaller.
As described above, galvannealed steel sheets with a plating layer consisting mainly of .delta..sub.1 phase approximating to an ideal type structure and having a low coating weight have been produced and provided to practical applications, but galvannealed steel sheets with a thick plating layer having a coating weight of at least 45 g/m.sup.2, which can satisfy both anti-powdering property and anti-flaking property and a process for producing the same have not been available yet and their development has been keenly desired.